Southern African Large Telescope. Prime Focus Imaging Spectrograph. Instrument Acceptance Testing Plan

Transcription

1 Southern African Large Telescope Prime Focus Imaging Spectrograph Instrument Acceptance Testing Plan Eric B. Burgh University of Wisconsin Document Number: SALT-3160AP0003 Revision April 2004

2 1 Scope The Instrument Acceptance Testing Plan describes the calibration activities, demonstrations and tests leading to acceptance of the Prime Focus Imaging Spectrograph (PFIS) consistent with the requirements of the instrument as defined in the Functional Performance Requirements Document (FPRD: SALT-3170AE0003). 2 Introduction This document describes the acceptance test plan for the Prime Focus Imaging Spectrograph for the Southern African Large Telescope. It contains the following components: A summary matrix listing all of the requirements with an indication of how each requirement will be verified. Descriptions of the procedures for each test to be performed at the University of Wisconsin to verify the instrument s compliance with the requirements as stated in the FPRD. Descriptions of the tests and inspections to be performed after arrival at the delivery location to confirm that PFIS continues to meet the specifications after transportation. Descriptions of the facilities and resources that will be needed at the delivery location to support assembly and testing of the instrument after it is delivered. 3 Verification Methods Matrix The matrix below relates each of the requirements listed in the FPRD to a method of verifying compliance described elsewhere in this document. FPRD Description Acceptance Test Req. Section Pre-Delivery Post-Delivery Field Size 5.1 Telescope test Slit-mask capability 5.2 Lab test at UW Lab Test at SAAO Collimation 5.3 Lab test at UW Lab Test at SAAO Image Quality 5.4 Lab test at UW Lab test at SAAO Telescope test Focus Range 5.5 Lab test at UW Lab Test at SAAO Detector Pixel Scale 5.6 Lab test at UW Telescope test Flexure 5.7 Lab test at UW Telescope test Transmission 5.8 Lab test at UW Telescope test Stray light 5.9 Lab test at UW Telescope test Field of view 6.1 Telescope test Max Resolution 6.2 Lab test at UW Telescope test Efficiency 6.3 Manufacturer s Spec Telescope test Central Wavelength Precision 6.4 Lab test at UW

3 2.3.1 Etalon Resolution 7.1 Lab test at RU Spectral Range 7.2 Lab test at RU Field of view 7.3 Telescope test Wavelength Gradient/Bullseye 7.4 Lab test at UW Wavelength Precision 7.5 Lab test at RU Wavelength Stability 7.6 Lab test at RU Wavelength Set Time 7.7 Manufacturer s Spec Efficiency 7.8 Lab test at RU Telescope test Parasitic Light 7.9 Lab test at RU Polarimetric Field of view 8.1 Telescope test Polarimetric Efficiency 8.2 Lab test at UW Telescope test Instrumental Polarization 8.3 Telescope test Position Angle Repeatability 8.4 Lab test at UW SAAO test Transmission 8.5 Lab test at UW Telescope test CTE 9.1 Lab test at SAAO Full Well 9.2 Lab test at SAAO Sensitivity 9.3 Lab test at SAAO Dark Current 9.4 Lab test at SAAO Lab test at SAAO Readout Noise 9.5 Lab test at SAAO Lab test at SAAO Gain 9.6 Lab test at SAAO Prebinning 9.7 Lab test at SAAO Readout Speed 9.8 Lab test at SAAO 4 Regarding SALT prerequisites A number of the tests listed in the above verification matrix indicate Telescope test. This requires that PFIS be mounted, aligned, and fully integrated onto SALT; however, not all of the tests will necessarily require on-sky time. To best perform these tests, and adequately ensure that the instrument has passed acceptance, there will be requirements placed on the performance of SALT in several categories. These include (with specs listed from SALT System Specification (SALT- 1000AS0007): Telescope pointing accuracy (PA) o <15 arcsec peak to peak to any accessible point in the sky after an azimuth move Telescope acquisition (ACQ) Telescope tracking performance (TRK) o Open-loop: maintain pointing accuracy o Closed-loop: <0.1 arcsec RMS over entire tracking range Optical Image Quality (IMG) o EE(50)<0.6 arcsec >90% of the time at 633 nm, not including natural seeing. o Image quality not degraded by more than 10% after 5 days since last Primary Mirror alignment Efficiency and quality of the calibration system(s) (CAL)

4 SALT Requirement Priority On Telescope Tests: (L = Low, M = Medium, H = High) PA ACQ TRK IMG CAL Field Size L L M M L Image Quality L L M M L Det. Pixel Scale M L M M L Flexure M L H M L Transmission M L H M H Stray Light L L M L M Grating FOV L L M M L Max Resolution M L H H M Grating Efficiency M L M M H FP FOV L L M M L FP Efficiency M L M M H Polarimetry FOV L L M M L Polarimetric Efficiency M L M M M Instrumental Polarization M L M M M Polarimetric Transmission M L M M M 5 Optics Requirements 5.1 Field Size REQ-FPR-2.1.1: Field size should be 8 arcminutes in diameter, except for limitations for polarimetry, high-speed, and nod and shuffle modes. Post-delivery compliance will be verified at the telescope. 5.2 Slit-mask capability REQ-FPR-2.1.2: Arbitrary slitmask features down to 0.45 arcsec; e.g arcsec slitlets. Pre-delivery compliance will be verified by lab test at UW. Multi-slit masks will be generated by the laser slitmask facility. Measurements of slit width, edge smoothness, and relative positions will be made using a microscope. 5.3 Collimation REQ-FPR-2.1.3: <60 micron defocus at detector 5 20º C.

5 Pre-delivery compliance will be verified by lab test at UW. Collimation will be determined during assembly and integration of collimator into the structure. An autocollimation setup will be used and is further described in the Optical Integration and Test Plan (SALT-3160BP0001). Post-delivery compliance will be verified by lab test at SAAO. Collimation will be determined by analyzing images of ghost reflection off of the grating, which resides in the collimated beam. Reflected ghosts will be in focus when at best collimation. 5.4 Image Quality REQ-FPR-2.1.4: See FPRD Table 1. Pre-delivery compliance will be verified by lab test at UW. Circular image quality verification will use a pinhole array, fabricated with the laser slitmask facility, and inserted into the focal plane by the slitmask mechanism. The pupil illumination lamp will be used and spots recorded and measured. This test will also be used to measure optical distortion. Slit response image quality verification will use a narrow longslit in the same manner. This will be performed for each grating at a few specified grating tilts. Post-delivery compliance will be verified by lab test at SAAO, before transport to SALT site. This test will verify survival of optics and integrity of the optical alignment postshipping. Same as above, except a smaller number of grating tilts will be tested. 5.5 Focus Range REQ-FPR-2.1.5: ±400 microns

6 Pre-delivery compliance will be verified by lab test at UW. Optimal focus position will be measured in each major configuration, including all gratings at various tilts, all waveplate combinations, and filters. Post-delivery compliance will be verified by lab test at SAAO. Same as above, but in a limited number of configurations. 5.6 Detector Pixel Scale REQ-FPR-2.1.6: Camera has F/2.2 beam with a resultant detector pixel scale of arcsec/pixel; median telescope images of 1.2 arcsec(fwhm) = 9.4 pixels. Pre-delivery compliance will be verified by lab test at UW. Slits of known widths will be cut with the laser slitmask facility. Pupil illumination lamp will produce filled-slit images at the detector plane. Measurement of slit width in pixels will be determined from the images. Focal plane plate scale will be assumed to be 4.5 arcsec/mm for conversion of slit width to angular size. Observations will be made of astronomical objects with known angular extent. Particularly good are open clusters with precise astrometry. This test will also allow for total telescope plus instrument distortion to be measured. 5.7 Flexure REQ-FPR-2.1.7: <0.1 arcsec/track in grating dispersion direction; <0.15 arcsec/track perpendicular to dispersion. Pre-delivery compliance will be partially verified by lab test at UW.

7 The lack of a rotating stage for the mounting of PFIS results in a full test not being possible at UW. However, we will be able to test spot positions at various mounting angles for PFIS on the mounting wedge described in Section Error! Reference source not found., allowing for the determination of any gross deviations from expected flexure specification. A bright star will be observed through a long track with a significant roll angle. Multiple short exposures will be taken, the centroid of the star measured, and image motion mapped through the track. 5.8 Transmission REQ-FPR-2.1.8: See FPRD Table 2. Pre-delivery compliance will be verified by test at UW. Optics throughput will be calculated using vendor-provided anti-reflection coating efficiencies. Witness samples of coatings will be requested. Record spectra of a smooth spectrum flux standard star with each grating. Calculate throughput by comparing total detected signal to source photon flux density. This test will provide total telescope plus instrument throughput and will be used to verify individual component efficiencies in various modes. 5.9 Stray Light REQ-FPR-2.1.9: Collimator/Camera ghost brightness <10-4 ; Disperser (focused) ghost brightness <10-3. Pre-delivery compliance will be verified by lab test at UW.

8 Pinhole slitmask will be inserted into the focal plane and illuminated by an emission line lamp. For each grating and various tilts, record a spectrum with exposure times such that the brightest lines are just below saturation. A comparison of line brightness to off-line brightness in both spatial and spectral direction will be measured. This test also serves as a ghost analysis. A bright star will be placed in the field and observed. Extended point spread function will be examined. This test will be performed with the bright star placed at various field locations. 6 Grating Spectroscopy Requirements 6.1 Field of view REQ-FPR-2.2.1: 8 arcmin diameter, limited in direction of dispersion by required spectral coverage. 6.2 Max Resolution REQ-FPR-2.2.2: 1.25 arcsec slit R=5300; 0.6 arcsec slit R= Pre-delivery compliance will be verified by lab test at UW. Slits of 0.9 arcsec (0.2 mm) and 0.45 arcsec (0.1 mm) will be cut using laser slitmask facility and inserted at the focal plane. Grating will be inserted and camera placed at full articulation. The pupil illumination lamp in conjunction with an emission-line source will produce filled-slit spectra at the detector plane. The FWHM of pertinent emission lines will be measured and resolution determined. On sky resolution will be examined using narrow slits on targets with strong, isolated absorption features, such as narrow interstellar absorption lines, as well as emission line sources, such as planetary nebulae. The combined SALT/PFIS line spread function will be determined.

9 6.3 Grating Efficiency REQ-FPR-2.2.3: See FPRD Table 4. Pre-delivery compliance will be verified by vendor. Testing by vendor will be requested before acceptance. The specifications for the VPH gratings are listed in document SALT-3120AS0015. Record spectra of a smooth spectrum flux standard star with each grating. Calculate throughput by comparing total detected signal to source photon flux density. This test will provide relative grating efficiencies for areas of spectral overlap, determining contribution of grating to overall throughput. 6.4 Central Wavelength Precision REQ-FPR-2.2.4: λ <1 nm x (300/σ) Pre-delivery compliance will be verified by lab test at UW. Using emission line lamp, spectra will be taken in various grating modes and tilts. The precision and repeatability of the central wavelength will be measured by returning to a specific configuration after changing gratings and articulation angles. 7 Fabry-Perot Spectroscopy Requirements 7.1 Etalon Resolution REQ-FPR-2.3.1: Etalon complement/resolution Low Res: Mid Res: High Res: R(Hα)= R(Hα)=2500 (blocked by Low Res) R(Hα)=12500 (blocked by Low Res) Compliance tests and procedure described in Section 3 of the Fabry-Perot Subsystem CDR document (SALT-3180AE0002).

10 7.2 Spectral Range REQ-FPR : Spectral range nm. Compliance tests and procedure described in Section 3 of the Fabry-Perot Subsystem CDR document (SALT-3180AE0002). 7.3 Field of View REQ-FPR-2.3.3: 8 arcmin diameter. Compliance tests and procedure described in Section 3 of the Fabry-Perot Subsystem CDR document (SALT-3180AE0002). 7.4 Wavelength Gradient/Bullseye REQ-FPR-2.3.4: Wavelength Gradient: λ r =λ c cos(4.877 x r/4 ); r=radial distance from center of field in arcmin. Bullseye: 1.3 x (10450/r) 1/2 (diameter of field where λ gradient = FWHM). Pre-delivery compliance will be verified by lab test at UW. Full field images of an emission line source, in conjunction with the pupil illumination lamp, will be taken. This should produce rings on the detector corresponding to the various emission lines. The Rutgers software, described in Section 4 of the Fabry-Perot Subsytem CDR Document (SALT-3180AE0002), will be used to produce the wavelength solution and gradient. 7.5 Wavelength Precision REQ-FPR-2.3.5: FWHM/50 (minimum). Compliance tests and procedure described in Section 3 of the Fabry-Perot Subsystem CDR document (SALT-3180AE0002). 7.6 Wavelength Stability REQ-FPR-2.3.6: FWHM/3 per hour (minimum).

11 Compliance tests and procedure described in Section 3 of the Fabry-Perot Subsystem CDR document (SALT-3180AE0002). 7.7 Wavelength Set Time REQ-FPR-2.3.7: 2 msec (maximum). Compliance tests and procedure described in Section 3 of the Fabry-Perot Subsystem CDR document (SALT-3180AE0002). 7.8 Efficiency REQ-FPR-2.3.8: 75% minimum (approximately achromatic); 80% expected (approximately achromatic). Compliance tests and procedure described in Section 3 of the Fabry-Perot Subsystem CDR document (SALT-3180AE0002). 7.9 Parasitic Light REQ-FPR-2.3.9: 1.5% Low Res (max), 1.0% Mid Res (max), 6.0% High Res (max) includes adjacent orders and regions outside coating range Compliance tests and procedure described in Section 3 of the Fabry-Perot Subsystem CDR document (SALT-3180AE0002). 8 Polarimetry Requirements 8.1 Field of view REQ-FPR-2.4.1: Half-height apertures will be used in polarimetric modes. 4x7.2 arcminute field (linear); 3 arcmin diameter (circular and all-stokes). 8.2 Efficiency REQ-FPR-2.4.2: Linear: >95%, calibrated to better than ±0.5%. Circular: >95%, calibrated to better than ±0.5%. Pre-delivery compliance will be verified by test at UW.

12 Polarimetric efficiencies will be measured separately for the individual waveplates and the beamsplitter by test in the integrated PFIS optical system. A continuum source will be fed into the pupil illumination system and a specialized slit-mask with a polarizer will be inserted into the focal plane. Images will be taken at different waveplate settings to determine efficiency. Polarimetric standard stars will be observed in linear and circular polarimetric modes. 8.3 Instrumental Polarization REQ-FPR-2.4.3: Linear: <0.4%, calibrated to <0.04%. Linear to circular: <3x10-3, calibrated to <3x10-4. Polarimetric standard stars will be observed in linear and circular polarimetric modes. 8.4 Position Angle Repeatability REQ-FPR-2.4.4: Repeatability <0.1 degree. Pre-delivery compliance will be verified by lab test at UW. Same procedure as for 8.2, monitoring consistency of efficiency at repeated waveplate positions. Post-delivery compliance will be verified by lab test at SAAO. Same as above.

13 8.5 Transmission REQ-FPR-2.4.5: 70% of spectroscopic/imaging modes at 650 nm. Pre-delivery compliance will be verified by lab test at UW. Transmission of the waveplates relative to the waveplate blanks will be measured by comparing the measured flux of a calibration lamp in the inserted and retracted waveplate positions. Absolute transmission of the beamsplitter will be measured by comparison of flux in the inserted and retracted positions. Unpolarized standard stars will be observed in polarimetric modes. Comparison of the total flux with imaging or spectroscopic modes will provide relative transmission measurement. 9 Detector Requirements 9.1 CTE REQ-FPR-2.5.1: CTE= % (typical), % (guaranteed). Pre-delivery compliance will be verified by lab test at SAAO before shipment of the detector subsystem to UW. Procedures for the testing of the detector subsystem are detailed in the Detector Package Testing and Commissioning Plan (SALT-3190AE0004). 9.2 Full Well REQ-FPR-2.5.2: 200 k e - /pix (typical) 150 k e - /pix (guaranteed). Pre-delivery compliance will be verified by lab test at SAAO before shipment of the detector subsystem to UW. Procedures for the testing of the detector subsystem are detailed in the Detector Package Testing and Commissioning Plan (SALT-3190AE0004). 9.3 Sensitivity

14 REQ-FPR-2.5.3: Thinned, back-side illuminated; deep depletion silicon; astro broad band anti-reflection coating. CCD QE, see FPRD Table 5. Pre-delivery compliance will be verified by lab test at SAAO before shipment of the detector subsystem to UW. Procedures for the testing of the detector subsystem are detailed in the Detector Package Testing and Commissioning Plan (SALT-3190AE0004). 9.4 Dark Current REQ-FPR-2.5.4: Dark current of 1 e - /pix/hr (typical) at 163 K. Pre-delivery compliance will be verified by lab test at SAAO before shipment of the detector subsystem to UW. Procedures for the testing of the detector subsystem are detailed in the Detector Package Testing and Commissioning Plan (SALT-3190AE0004). 9.5 Readout Noise REQ-FPR-2.5.5: 3.0 e - /pix at 100kHz (10.0 µsec/pix) TBC4; 5.0 e - /pix at 345 khz (2.9 µsec/pix) TBC4. Pre-delivery compliance will be verified by lab test at SAAO before shipment of the detector subsystem to UW. Procedures for the testing of the detector subsystem are detailed in the Detector Package Testing and Commissioning Plan (SALT-3190AE0004). 9.6 Gain REQ-FPR-2.5.6: Software selectable from : x1; x2; x4.75; x9.5. Pre-delivery compliance will be verified by lab test at SAAO before shipment of the detector subsystem to UW. Procedures for the testing of the detector subsystem are detailed in the Detector Package Testing and Commissioning Plan (SALT-3190AE0004). 9.7 Prebinning REQ-FPR-2.5.7: 1x1 to 9x9, independently in each direction. Pre-delivery compliance will be verified by lab test at SAAO before shipment of the detector subsystem to UW.

15 Procedures for the testing of the detector subsystem are detailed in the Detector Package Testing and Commissioning Plan (SALT-3190AE0004). 9.8 Readout Speed REQ-FPR-2.5.8: Frame transfer architecture: sec frame transfer time khz ( µsec/pix) TBC5. See FPRD Table 6 for detector readout times. Pre-delivery compliance will be verified by lab test at SAAO before shipment of the detector subsystem to UW. Procedures for the testing of the detector subsystem are detailed in the Detector Package Testing and Commissioning Plan (SALT-3190AE0004).

16 10 Characterization Tests Characterization tests are not required to be performed in order to pass the instrument acceptance as defined by the requirements in the FPRD. However, given enough time to perform them, these tests will greatly aid our understanding of the instrument as well as verify the expected performance and find any residual mechanical issues that may need resolving. The tests are listed below, grouped in priority, with descriptions of their goals and procedures Baffling/stray light Priority: High This will test the extent of the light-proofing of the baffling system. An empty slitmask will be put in place covering any light from entering through the focal plane. Dark CCD frames will then be taken with the room in various states of illumination. Additionally, spot light sources will be placed at various positions in the room, attempting to simulate glints Image wander with waveplate rotation Priority: High This tests measures any residual waveplate tilt. If there is a tilt in the waveplate normal relative to the optical axis, there will be image motion at the detector plane with correlated with the rotation of the waveplate. A tilt of about 6 arcminutes produces a motion of half a pixel over a full rotation. Use on-axis pinhole and light source. Measure spot centroid for at least 8 separate waveplate positions. Repeat for each waveplate Distortion Priority: High Distortion is necessary to measure in order to produce accurate slitmasks from PFIS preimaging. Also, a measurement of it will aid in the extraction of multi-slit spectra from CCD data. We expect pincushion distortion of about 2%. The distortion information will be obtained from the image quality testing described above while using a pinhole array slitmask.

17 10.4 Thermal testing Priority: High Thermal testing will allow us to verify the focus model as well as measure any changes in focal plane scale and image quality with temperature. We do not have a controlled environment for thermal testing and thus will rely on simple environmental control of the air conditioning to raise or lower the temperature in the room. The PFIS temperature sensors will be used to measure and verify the stability of the room temperature. A pinhole array slit mask will be used and the best focus position measured Pupil Masking Priority: medium Pupil masking tests will allow us to examine any efficiency variations across the surface of the gratings. Additionally, we can measure the throughput differences for the individual prisms in the beamsplitter array. Use a mask at the entrance pupil position (surface of the backlight or fresnel lens in the calibration setup) to isolate specific regions of the pupil.

18 10.6 Field dependence of waveplates Priority: medium The spectro-polarimetric efficiency of the waveplates may be dependent on the incoming field angle. For the axis perpendicular to dispersion a longslit will be put in place. Spectropolarimetry of the calibration lamp through a polarizer will be performed. Variations across the longslit will be measured (corrected as much as possible for vignetting variations in the collimator, if any). For the dispersion axis, a mask will be made with short slits that will produce non-overlapping spectra, keeping in mind that this will mix variations in both axes Grating blaze shift with field angle Priority: medium A peculiar feature of VPH gratings is that the wavelength dependence of the efficiency for a given grating angle varies with field angle. Using the calibration setup with the continuum lamp, record a spectrum at 0, 2, and 4 arcminutes off-axis in the dispersion direction. A relative performance comparison can be made versus the on-axis efficiency Off-littrow G0900 performance Priority: medium According to Rigorous Couple Wave Analysis of the performance of the G0900 grating, it is possible that the efficiency will be greater in an off-littrow configuration. This test will determine the best grating angle for various G0900 camera positions. For various camera angles, adjust the G0900 grating angle and record a spectrum. Compare the spectral intensity to the littrow condition and determine best angle. If the performance variations are smooth enough, interpolation of the best angle for intermediate camera positions can be calculated Absolute throughput

19 Priority: medium Use photodiode and lamp to measure brightness of lamp. Then make setup that makes sure the same light goes through the pinhole Vignetting Priority: low The vignetting of the optical system can only be measured to the accuracy of the flatness and beam speed of the pupil illumination of the calibration setup. This will allow, however, for identification of any gross deviations from uniformity. Use full field slitmask and image the pupil onto the detector. Further, if the pupil illumination lamp is bright enough, the pupil at the grating can be examined by placing a white sheet of paper in the beam at this point Shutter latency Priority: low Deviations from linearity between the actual shutter-open time and the commanded shutter-open time will be examined. If they exist they would become apparent at short exposure times. Take several images at the same exposure time first to examine the stability of the brightness of the calibration lamp. Once that is reliably measured, take a series of images with decreasing shutter time. Measure count level on the detector (after correcting for bias and potential dark contribution) and divide by exposure time. Deviations from linearity will show up as an increase in count rate at short exposure times. 11 Post-delivery inspection and testing 11.1 Transportation configuration To ensure survival of transportation, the optics will not travel attached to the PFIS structure. The optics will be separated into several sections: the field lens assembly, the main collimator optics barrel, the collimator doublet, the folding flat mirror, and the camera tube with optics installed.

20 11.2 Post-delivery assembly and alignment Upon arrival at SAAO in Cape Town, the optics will be reassembled onto the PFIS structure, and alignment testing will be performed. This will follow the alignment procedure described in the Optical Integration and Test Plan (SALT-3160BP0001) Post-delivery inspection and testing As described in the requirements above, certain tests will be repeated in the SAAO lab in Cape Town before transport of PFIS to the SALT site. These will primarily be for verification that PFIS has survived transportation and continues to meet the requirements as it did pre-shipping. 12 Mechanical verification In addition to the requirements listed above, specifications for the performance of each mechanism have been called out in the various mechanism documents (SALT- 3130AE0501 et al.). Some of these mechanisms can be tested through bench test, without needing to be integrated into the PFIS structure: the slit mask mechanism, waveplate mechanism, and grating rotation. The Pilot Group will test the focus mechanism during optical assembly. The grating insertion mechanism, beam splitter insertion mechanism, Fabry-Perot insertion mechanism, articulation mechanism, and filter mechanism will not be fully tested until integrated into PFIS. 13 Operational verification A procedure for operational verification will be developed. This will include a series of commands driving the various mechanisms into different states to verify liveliness and functionality.